Range from a reference point or plane to a given target point can be measured in a variety of ways. Passive techniques, such as stereo or range-from-focus, require illuminated target surfaces and typically require complexly patterned surfaces for reliable results. Active echoing techniques, such as RADAR, SONAR and LIDAR employ emitted electromagnetic, acoustic, and light energy, and monitor the reflected energy from the target surface. These techniques use a time-of-flight measurement as a basis for determining range and are typically expensive and complex.
In machine vision research and application, a different active technique called structured illumination has been employed. A ray of light is directed to the target surface along a direction which is not coaxial with the optic axis of the one or two dimensional sensing device. The intersection of the ray and target surface produces a spot of light which is imaged on the sensing plane of the imaging device. The 3-D position of this spot in space may be calculated from the known position and orientation of the imaging device from basic trigonometric relations. This structured illumination technique is called triangulation.
FIGS. 1a and 1b illustrate simple laser triangulation devices. With the geometry illustrated in FIGS. 1a and 1b, including a laser 10, a lens 11, a detector 12 and an object 13, a reflected spot is imaged higher on the detector 12 as the target surface moves farther from the imaging lens. In most devices using this geometry, however, the reflected spot is in focus for only one target distance and is blurred in varying degrees for all other distances in the depth of field as shown in FIGS. 2a and 2b. Blurring reduces the sensitivity of the device and effectively reduces the depth of field, i.e., the range over which a system can provide satisfactory definition.
Three of the basic performance measures of a ranging system are standoff distance, depth of field, and range resolution. The standoff distance is the nominal range of the device and is typically arbitrarily chosen as the near point, mid point or far point in the depth of field. The resolution is the smallest change in range values the system can discriminate and typically varies over the depth of field. Triangulation devices are engineered for a certain standoff distance, depth of field, and range resolution. That is, the geometry of a device is chosen based on the application, and desired changes in performance require changing imaging optics. In certain applications, such as imaging in remote areas, such inflexibility is unacceptable.
A first problem to be solved, therefore, is to provide blur-free imaging on a detector so that all reflected target returns are in focus. A second problem to be overcome is to construct a sensor that is reconfigurable in the sense that standoff distance, depth of field, and/or range resolution can be varied under electronic control without requiring the replacement of components.
It is well known in the photographic industry that tilting the camera at the moment of exposure leads to an effect called keystoning, in which parallel lines in the subject appear as converging lines in the result. This can be rectified during printing by tilting the enlarger easel by the same angle by which the camera was tilted. To achieve sharp imagery throughout the result, the enlarger lens must also be tilted slightly so that the planes of the lens, easel, and negative all meet at a common location, in accordance with what is known as the Scheimpflug condition in optics. The Scheimpflug condition can also be interpreted in an alternative way: a line on the object side of the lens is imaged to a line on the image side of the lens, and the two lines intersect with the line representing the plane of the lens, as shown in FIG. 3a. If the ray of light for the system is directed along some line on the object side of the lens, then the detector should be disposed along the Scheimpflug condition-predicted line on the image side to facilitate blur-free imaging of reflected target returns as shown in FIG. 3b.
The three system parameters--standoff distance, depth of field, and range resolution--can be calculated if the geometry of the triangulation ranging system is known as well as information about the imaging lens and detector. Conversely, any two of these three performance measures may be selected and the system geometry can be determined given a specific imaging lens and detector. Providing the mechanical degrees of freedom in such a sensor results in a reconfigurable system capable of a wide range of performance measures without requiring the replacement of the physical components.
A variable depth triangulation ranging system is described in allowed and copending U.S. patent application Ser. No. 07/345,750, Variable Depth Range Camera, which is assigned to the present assignee. The ranging system includes a light beam emitting component to generate and direct a light ray towards an object, a photodetector component capable of measuring distance along a single axis, i.e. a linear photodetector, and an imaging lens component for imaging light reflected off the object onto the detector. An image line is defined that extends longitudinally through the photodetector, and a plane axis that passes through the lens component. One of the three system components is at a fixed location, and means are provided for adjustably positioning the other two components such that the light beam, plane axis and image line all intersect approximately at a common point and satisfy the Scheimpflug condition. The two movable components are reconfigurable whereby the values of any two performance parameters selected from the group consisting of standoff distance, depth of field, and range resolution at a point within the depth of field may be chosen and the system geometry changed to achieve those values. The system includes means to calculate system geometry and range from received detector signals. A scanner may be provided to scan the light beam along a line or over an area. The light source may be a laser and the photodetector may be a lateral effect photodiode, linear array sensor or position sensitive photomultiplier.
In the above-described system, it has been found that it is preferred that the light beam be focused into a small spot on the target. This facilitates improving the resolution and accuracy with which the receiving optics can collect reflected light from the target and focus it onto a detector. Maintaining a small spot on the target as the target is scanned requires real-time, dynamic control of laser spot size.
Also, in order to facilitate high resolution, it is preferred that the detector be long with a large number of effective elements. The resolution and accuracy of the detector improves performance of the overall system.
Moreover, as discussed above, the position of the lens relative to the detector may have to be adjusted to obtain a satisfactory resolution. For some lens focal lengths, however, a required lens-to-detector distance is not suitable. For example, the lens-to-detector distance may be more than ten feet for some applications.
It is therefore an object of the present invention to provide a system for real-time, dynamic control of laser spot size.